Effect of ultrasonic field on pool boiling heat transfer to Al2O3 nanofluid on ribbed surfaces

Nanofluid was frequent technique which solved microscale heat and mass transport problems to intensify pool boiling heat transfer. This paper had investigated the effect of nanofluid, ultrasonic field and ribs surfaces on cooperative heat transfer enhancement in pool boiling. A two-step method was applied to prepare Al2O3 nanofluids of different kinds of concentrations and diameters. Group experiments were examined the influences of nanofluid concentration and diameter, ultrasonic power and distance and rib surface type on heat transfer characteristics, respectively. The heat flux and convection heat transfer coefficient were used the performance parameters. The results showed that the concentration and diameter of the Al2O3 nanofluid effected the heat transfer performance on the pool boiling. The Al2O3 nanofluid with an average diameter of 80 nm had the most significant enhancement effect on the pool boiling heat transfer performance, but the Al2O3 with a diameter of 30 nm had the weakest enhancement effect, when the concentration of Al2O3 nanofluid was 0.01 wt%. Meanwhile, the generated acoustic streaming effect and cavitation effect by ultrasonic field can promote the energy accumulation, rupture and regeneration of the bubbles at low superheat stage on nanofluid pool boiling. The heat transfer coefficient (HTC) gradually enhances as the ultrasonic field changes. At an ultrasonic power of 528 W and a distance of 50 mm, the nanofluid pool boiling increased by 44.34% compared to no ultrasonic field HTC. Compared with No rib, the boiling point of the Al2O3 nanofluidic pool boiling of Rib II was reduced by 2°C under the ultrasonic field. The nanofluid pool boiling with ultrasonic field of p = 528 W and Rib II had the best heat transfer effect, and HTC improved more than 50%. This is due to the addition of more nucleation points for bubbles on the heated surface of the Rib II.